162 



SIR H. DAVY S AGRICULTURAL CHEMISTRY. 



coflfee is preferable. By means of a pro- 

 portionate quantity of pure soda, a f2;reen 

 precipitate is obtained, which is suffered 

 to dry for six or seven days upon polish- 

 ed marble, stirritif^ it about occasionally 

 in order that every part may come in 

 contact with the atmosphere, from which 

 it receives a new vivacity of tint. Nei- 

 ther the acids, light nor moisture, have 

 power to injure this green lake. 



Reg. Art. Sci. 



SIR H. D AVY S AGRICULTURAL CHEMISTRY. 



(Continued from p. 153.) 



Table covtinued from p. 153. 



Besides the principles, the nature of 

 which has been just discussed, others 

 have been described by chemists as be- 

 longing to the vegetable kingdom. Thus 

 a substance somewhat analogous to the 

 muscular fibre of animals has been de- 



tected by Vauquelin in the papavv; and 

 a matter similar to animal gelatine by 

 Braconnot, in the mushroom; but in this 

 place it would be improper to dwell upon 

 peculiarities, my object being to offer 

 such general views oif the constitution of 

 vegetables, as may be of use to the agri- 

 culturist. Some distinctions have been 

 adopted by systematical authors, which I 

 have not entered into, because they do 

 not appear to me essential to this inquiry. 

 Dr. Thomson, in his elaborate and learn- 

 ed system of chemistry, has described six 

 vegetable substances, which he calls mu- 

 cus, jelly, sarcocol, asparagin, inulin, and 

 ulmin. He states that mucus exists in 

 its purest form in linseed; but Vauquelin 

 has lately shown that the mucilage of lin- 

 seed is, in its essential characters, analo- 

 gous to gum; but that it is combined with 

 a substance similar to animal mucus; 

 vegetable jelly, Dr. Thomson himself 

 considers as a modification of gum. It is 

 probable, from the taste of sarcocol, that 

 it is gum combined with a little sugar. 

 Inulin is so analogous to starch, that it is 

 probably a variety of that principle; ul- 

 min has been lately shown by Mr. Smith- 

 son to be a compound of a peculiar ex- 

 tractive matter and potassa; and asparagin 

 is probably a similar combination. If 

 slight differences in chemical and physi- 

 cal properties be considered as sufficient 

 to establish a difference in the species of 

 vegetable substances, the catalogue of 

 them might be enlarged to almost any 

 extent. No two compounds procured 

 from different vegetables are precisely 

 alike; and there are even differences in 

 the qualities of the same compound, ac- 

 cording to the time in which it has been 

 collected, and the manner in which it has 

 been prepared: the great use of classifi- 

 cation in science is to assist the memory; 

 and it ought to be founded upon the si- 

 milarity of properties which are distinct, 

 characteristic, and invariable. 



The analysis of any substance contain- 

 ing mixtures of the different vegetable 

 principles, may be made in such a man- 

 ner as is necessary for the views of the 

 agriculturist, with facility. A given 

 quantity, say 200 grains, of the substance 

 should be powdered, made into a paste 

 or mass, with a small quantity of water, 



